This seems a little bit convoluted. Since phys_to_dma and dma_to_phys
will always end up calling the loongson_unity_ functions, I don't see
any point in having the loongson_dma_map_ops structure to point to them.
You can just implement phys_to_dma and dma_to_phys as inlines in
mach-loongson/dma-coherence.h.